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The Book of Happenstance has, at the outset, the feel of a mystery, and not just because the first chapter opens with a death, the second with a theft.
Much of the novel consists of actual ongoing investigations -- into the crime, but also into such subjects as the nature of the universe and life itself, as the narrator repeatedly turns to a colleague for explanations of the Big Bang, DNA, and the like.
The narrator, Helena Verbloem, works as a lexicographer, recently hired to assist Theo Verwey on his grand project:

to gather into a single book all the words that have become obsolete in Afrikaans, all expressions no longer in common use

Verwey is the man whose death is announced in the second sentence of the novel, but most of the novel is retrospective, recalling what took place earlier that year,
as Helena describes her life and work and workplace during her time working for Verwey.
She is also, incidentally, working on a novel -- and obsesses about her lost sea shell, the only things stolen from her
in a burglary.
There are also reminders of (and reflections on) her past, including in the repeated phone calls from a man who met her almost three decades earlier (but whom she seems to have no recollection of), a conscience from an earlier time (which she doesn't much like listening to).
The account is precise and direct, and Helena straightforward and open -- yet much is only revealed piecemeal, and much that might be considered significant (about family members, her lover, her previous book) is often mentioned only casually, or deep into the narrative (even as she harps on her shells, or questions of etymology).
The account is also filled with repetition, proceeding in loops, returning again and again to exposition of the music they listen to while working, etymological and philosophical discussions, the sea shells, the mysterious phone calls from the man from her past, reminiscences about her much older sister.
The narrator suggests:

I could divide my life into five phases.
Childhood.
Turbulent youth.
Misguided twenties and early thirties.
The unhappy phase of my ill-fated marriage.
And at present the advent of the final phase: the beginning of the end.

Only slowly does a full picture of these phases emerge (and some remain clouded to the end); she may be in her final phase, but she still is burdened by much from her past (and indeed looks more towards that than any future (or fatalistic end)).
The Book of Happenstance is an often fascinating puzzle of this narrator, and the people around her, Helena's sustained intense voice making for a narrative tension that grabs hold from the first.
The severe tone and detached observation do often deal with the emotional, but she's careful as to what she will open up about (the sea shells, yes; her love life, not so much).
She's willing to expose herself -- but entirely on (and in) her own terms:

I do not see myself as heartless.
I have a particularly heavy and sorrowful heart: dark, heavy, and saturated with blood.
A hairy sack, like the sun in Revelations.

The air of mystery is sustained throughout, most obviously in the odd story of the fate of her shells, but also in most other respects -- and in a way this also undoes the novel, because Winterbach can't (emotionally) satisfyingly bring about a resolution: even at the end, most of the tension remains, leaving a book that is dangerously brittle.
What is for long stretches a riveting read ultimately can't fulfill expectations.
Winterbach is a remarkable stylist -- the voice, and much of the presentation impresses tremendously -- but the many pieces and the structure demand a more worked-out resolution.